/Engine/Source/Runtime/MovieScene/Public/EntitySystem/MovieSceneEntityManager.h
the array will not be emptied by this function - that is the responsibility of the callee, if desired.
void GetImmediateChildren ( [FMovieSceneEntityID](API\Runtime\MovieScene\EntitySystem\FMovieSceneEntityID) ParentID, [TArray](API\Runtime\Core\Containers\TArray)< [FMovieSceneEntityID](API\Runtime\MovieScene\EntitySystem\FMovieSceneEntityID) > & OutChildren ) const